Heat sinks including nonlinear passageways

ABSTRACT

A stereolithographically fabricated heat sink may include non-linear, or convoluted passageways therethrough, through which air can flow. The heat sink may also include a heat dissipation element that is configured to release heat as air flows past a surface thereof. As at least a portion of the heat sink is stereolithographically fabricated, that portion can have a series of superimposed, contiguous, mutually adhered layers of thermally conductive material.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/502,107, filed Feb. 10, 2000, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to heat sinks used todissipate heat from semiconductor devices during normal operationthereof. Particularly, the present invention pertains to the use ofstereolithographic techniques to fabricate heat sinks for use onsemiconductor devices, to heat sinks so fabricated, and to semiconductordevices including stereolithographically fabricated heat sinks.

[0004] 2. State of the Art

Heat Sinks

[0005] During normal use, semiconductor devices generate heat. Adequatedissipation of the heat generated during normal use of a semiconductordevice is necessary for the safe and reliable operation of an electronicappliance that includes the semiconductor device. If the semiconductordevice reaches an excessively high temperature, the integrated circuitsof the semiconductor device can fail or a circuit board fire can result,damaging the electronic system of which the semiconductor device is apart.

[0006] While some semiconductor devices are able to dissipate sufficientamounts of heat without an additional heat sink or heat spreader, stateof the art semiconductor devices with increased speed, circuitcomplexity, and circuit density often require added heat sinks.

[0007] In particular, as semiconductor devices have become more dense interms of electrical power consumption per unit volume, heat generationhas greatly increased, requiring package construction which dissipatesthe generated heat much more rapidly. As the state of the artprogresses, the ability to adequately dissipate heat is often a severeconstraint on the size, speed, and power consumption of an integratedcircuit design.

[0008] In this application, a heat sink will be distinguished from a“heat spreader,” the former pertaining to a structure with a heattransfer portion or element positioned proximate to a semiconductordevice and a heat dissipation portion or element relatively more remotefrom the semiconductor device, the latter pertaining to a member whichchannels heat from a semiconductor die to leads which exit the diepackage. However, a heat sink and a heat spreader may together be usedto cool a device.

[0009] Typically, heat sinks are fabricated from materials with goodthermal conductivity, such as metals (e.g., aluminum, copper alloys,etc.), ceramic materials, and glass. The heat transfer portion of a heatsink is configured to absorb heat from the semiconductor deviceproximate thereto and, therefore, generally contours to at least aportion of a surface of the semiconductor device. The heat dissipationportion of a heat sink may include a series of small protrusions, whichare typically referred to as “fins,” that receive heat from the heattransfer portion of the heat sink and are configured to dissipate theheat away from the semiconductor device as air flows between the fins.The shapes, sizes, arrangement, spacing, and numbers of fins on a heatsink are configured so as to optimize the heat dissipation capabilitiesof the heat sink with respect to the particular heat dissipation needsof a specific type of semiconductor device.

[0010] Heat sinks are typically manufactured separately from thesemiconductor devices to which they are subsequently secured.

[0011] Conventionally, metal heat sinks have been manufactured byextrusion or casting processes. When extruded, molten metal is forcedthrough an extrusion die to produce an elongated extrusion of across-section taken transverse to the length thereof of a desired heatsink configuration. The elongate extrusion is then sectioned transverseto the length thereof to provide the heat sinks. Cast heat sinks aremanufactured by disposing a molten quantity of heat conductive materialinto a refractory mold.

[0012] Heat sinks can also be machined from blocks of material. Asconventional heat sinks have spaced apart fins, however, machiningprocesses waste a considerable amount of material. In addition, due tothe small size and high complexity of conventional heat sinks, the useof machining processes can be very time consuming and expensive. Forthese reasons, the use of machining processes to manufacture heat sinksis somewhat undesirable.

[0013] The use of extrusion, casting, and machining processes tomanufacture heat sinks are also somewhat undesirable since each of theprocesses limit the possible configurations of the manufactured heatsinks. For example, when extrusion is used, the transverse cross-sectiontaken along the entire length of each heat sink has the sametwo-dimensional shape, being that imparted by the two-dimensionalconfiguration of the extrusion die. When heat sinks are cast, theconfigurations thereof are determined by the casting molds. Typically,molds have two parts, and may include additional inserts to facilitatethe formation of more complex features. State-of-the-art machiningprocesses are limited to, at most, seven axes. Typically, however, lesscomplex three-axis or five-axis machines are used. Nonetheless, certaintypes of features, such as internally confined cavities and non-linearchannels cannot be formed easily when casting or state-of-the-artmachining equipment is used.

[0014] An alternative method for manufacturing heat sinks is disclosedin U.S. Pat. No. 5,814,536, issued to Rostoker et al. on Sep. 29, 1998(hereinafter “the '536 patent”). The '536 patent discloses the use ofpowder metallurgy techniques to form a heat sink. Thus, the heat sink isformed from a mixture of powdered metal (e.g., copper, aluminum,tungsten, titanium, and alloys thereof) and a suitable binder. Themixture is placed into a mold, where the metal particles are bonded toadjacent particles, or sintered together, under appropriate pressure andat an appropriate temperature. The binder, if any, is removed (i.e.,burned off) during the sintering process. The sintered heat sink canthen be machined to provide features that may not be readily obtained orpossible to obtain by the sintering process alone. Since the sinteringprocess of the '536 patent employs a mold, it is somewhat undesirabledue to the previously mentioned conformational limitations that arepresent when a mold is used.

[0015] As noted above, a prefabricated heat sink is conventionallyassembled with a semiconductor device. The assembly can then be packagedby known techniques, such as by transfer molding of a particle-filledpolymer, as known in the art. If such an assembly is packaged, however,the packaging mold must usually be configured so as to receive at leasta portion of the heat sink to permit its projection beyond the polymerpackaging. The manufacture of molds configured to receive heat sinks issomewhat undesirable due to the complexity of the mold designs and thehigh costs of machining such molds.

[0016] The art does not teach a method of fabricating heat sinks onsemiconductor devices or of fabricating heat sinks by stereolithography,or layered manufacturing, processes.

Stereolithography

[0017] In the past decade, a manufacturing technique termed“stereolithography,” also known as “layered manufacturing,” has evolvedto a degree where it is employed in many industries.

[0018] Essentially, stereolithography as conventionally practicedinvolves utilizing a computer to generate a three-dimensional (3-D)mathematical simulation or model of an object to be fabricated, suchgeneration usually effected with 3-D computer-aided design (CAD)software. The model or simulation is mathematically separated or“sliced” into a large number of relatively thin, parallel, usuallyvertically superimposed layers, each layer having defined boundaries andother features associated with the model (and thus the actual object tobe fabricated) at the level of that layer within the exterior boundariesof the object. A complete assembly or stack of all of the layers definesthe entire object, and surface resolution of the object is, in part,dependent upon the thickness of the layers.

[0019] The mathematical simulation or model is then employed to generatean actual object by building the object, layer by superimposed layer. Awide variety of approaches to stereolithography by different companieshas resulted in techniques for fabrication of objects from both metallicand non-metallic materials. Regardless of the material employed tofabricate an object, stereolithographic techniques usually involvedisposition of a layer of unconsolidated or unfixed materialcorresponding to each layer within the object boundaries, followed byselective consolidation or fixation of the material to at least apartially consolidated, fixed, or semisolid state in those areas of agiven layer corresponding to portions of the object, the consolidated orfixed material also at that time being substantially concurrently bondedto a lower layer of the object to be fabricated. The unconsolidatedmaterial employed to build an object may be supplied in particulate orliquid form, and the material itself may be consolidated or fixed, or aseparate binder material may be employed to bond material particles toone another and to those of a previously-formed layer. In someinstances, thin sheets of material may be superimposed to build anobject, each sheet being fixed to a next lower sheet and unwantedportions of each sheet removed, a stack of such sheets defining thecompleted object. When particulate materials are employed, resolution ofobject surfaces is highly dependent upon particle size, whereas when aliquid is employed, surface resolution is highly dependent upon theminimum surface area of the liquid which can be fixed and the minimumthickness of a layer that can be generated. Of course, in either case,resolution and accuracy of object reproduction from the CAD file is alsodependent upon the ability of the apparatus used to fix the material toprecisely track the mathematical instructions indicating solid areas andboundaries for each layer of material. Toward that end, and dependingupon the layer being fixed, various fixation approaches have beenemployed, including particle bombardment (electron beams), disposing abinder or other fixative (such as by ink-jet printing techniques), orirradiation using heat or specific wavelength ranges.

[0020] An early application of stereolithography was to enable rapidfabrication of molds and prototypes of objects from CAD files. Thus,either male or female forms on which mold material might be disposedmight be rapidly generated. Prototypes of objects might be built toverify the accuracy of the CAD file defining the object and to detectany design deficiencies and possible fabrication problems before adesign was committed to large-scale production.

[0021] In more recent years, stereolithography has been employed todevelop and refine object designs in relatively inexpensive materials,and has also been used to fabricate small quantities of objects wherethe cost of conventional fabrication techniques is prohibitive for same,such as in the case of plastic objects conventionally formed byinjection molding. It is also known to employ stereolithography in thecustom fabrication of products generally built in small quantities orwhere a product design is rendered only once. Finally, it has beenappreciated in some industries that stereolithography provides acapability to fabricate products, such as those including closedinterior chambers or convoluted passageways, which cannot be fabricatedsatisfactorily using conventional manufacturing techniques. It has alsobeen recognized in some industries that a stereolithographic object orcomponent may be formed or built around another, pre-existing object orcomponent to create a larger product.

[0022] However, to the inventor's knowledge, stereolithography has yetto be applied to mass production of articles in volumes of thousands ormillions, or employed to produce, augment or enhance products includingother, pre-existing components in large quantities, where minutecomponent sizes are involved, and where extremely high resolution and ahigh degree of reproducibility of results is required. In particular,the inventor is not aware of the use of stereolithography to fabricateheat sinks for use with semiconductor devices. Furthermore, conventionalstereolithography apparatus and methods fail to address the difficultiesof precisely locating and orienting a number of pre-existing componentsfor stereolithographic application of material thereto without the useof mechanical alignment techniques or to otherwise assuring precise,repeatable placement of components.

SUMMARY OF THE INVENTION

[0023] According to one aspect, the present invention includes a methodfor fabricating heat sinks for use with semiconductor devices. In apreferred embodiment of the method, a computer-controlled, 3-D CADinitiated process known as “stereolithography” or “layeredmanufacturing” is used to fabricate the heat sinks. Whenstereolithographic processes are employed, a heat sink is formed as aseries of superimposed, contiguous, mutually adhered layers of material.

[0024] As it is important that heat sinks absorb heat from a proximatesemiconductor device and dissipate the heat, the heat sinks of thepresent invention are preferably manufactured from materials that aregood heat conductors. Accordingly, the stereolithography processes thatare preferred for fabricating the heat sinks of the present inventionare capable of fabricating structures from materials with good thermalconductivity.

[0025] In one such stereolithography process, known as “selective lasersintering” or “SLS,” structures are fabricated from layers of powderedor particulate material. The particles in selected regions of each ofthe layers can be bonded together by use of a laser under the control ofa computer. The laser either heats the material particles and sintersadjacent particles together, heats a binder material mixed in with theparticles to bond the particles, or heats a binder material with whichthe material particles are coated to secure adjacent particles in theselected regions of a layer to one another.

[0026] Another exemplary stereolithography process that may be used tofabricate heat sinks incorporating teachings of the present invention isreferred to as “laminated object manufacturing” or “LOM.” Laminatedobject manufacturing involves the use of a laser or other cutting deviceto define the peripheries of a layer of an object from a sheet ofmaterial. Adjacent layers of the object are secured to one another toform the object.

[0027] The stereolithographic heat sink fabrication method of thepresent invention preferably includes the use of a machine vision systemto locate the semiconductor devices or substrates upon which heat sinksare to be fabricated, as well as the features or other components on orassociated with the semiconductor devices or substrates (e.g., bondwires, leads, etc.). The use of a machine vision system directs thealignment of a stereolithography system with each semiconductor deviceor substrate for material disposition purposes. Accordingly, thesemiconductor devices or substrates need not be precisely mechanicallyaligned with any component of the stereolithography system to practicethe stereolithographic embodiment of the method of the presentinvention.

[0028] In a preferred embodiment, the heat sink to be fabricated upon asemiconductor device component in accordance with the invention isfabricated using precisely focused electromagnetic radiation in the formof a laser under control of a computer and responsive to input from amachine vision system, such as a pattern recognition system, to defineeach layer of the object to be formed from a layer of material disposedon the semiconductor device or substrate.

[0029] According to another aspect, the present invention includesstereolithographically fabricated heat sinks, as well as semiconductordevices that include stereolithographically fabricated heat sinks. Asstereolithographic processes are used to fabricate these heat sinks, theheat sinks may be formed with features that cannot be defined by use ofconventional extrusion, sintering, or machining processes.

[0030] Other features and advantages of the present invention willbecome apparent to those of skill in the art through consideration ofthe ensuing description, the ensuing description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0031]FIG. 1 is a side view of a semiconductor device with a heat sinkembodying teachings of the present invention secured to a surfacethereof;

[0032]FIG. 2 is a cross-section taken along line 2-2 of FIG. 1;

[0033]FIG. 3 is a side view of a semiconductor device with another heatsink embodying teachings of the present invention secured to a surfacethereof;

[0034]FIG. 4 is a cross-section taken along line 4-4 of FIG. 3;

[0035]FIG. 5 is a top view of the semiconductor device shown in FIGS. 3and 4;

[0036]FIG. 6 is a partial perspective view of a semiconductor wafer withunsingulated semiconductor devices having heat sinks fabricated on thebacksides thereof;

[0037]FIG. 7 is a schematic representation of an exemplarystereolithography apparatus, a selective laser sintering apparatus, thatcan be employed in the method of the present invention to fabricate heatsinks on semiconductor devices or other substrates in accordance withthe method of the present invention;

[0038]FIG. 8 is a schematic representation of another exemplarystereolithographic apparatus, a laminated object manufacturingapparatus, that can be employed in the method of the present inventionto fabricate heat sinks in accordance with the method of the presentinvention;

[0039]FIG. 9 is a partial cross-sectional side view of a semiconductordevice or substrate disposed on a platform of a stereolithographicapparatus and depicting a heat sink being fabricated on thesemiconductor device or substrate; and

[0040]FIG. 10 is a cross-sectional view of another embodiment of a heatsink according to the present invention, depicting the heat sinkdisposed adjacent a surface of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION Heat Sinks

[0041] According to one aspect, the present invention includes heatsinks and assemblies including a semiconductor device and a heat sink.The heat sinks of the present invention are stereolithographicallyfabricated, or layer-manufactured. Thus, heat sinks incorporatingteachings of the present invention have a plurality of superimposed,contiguous, mutually adhered layers of heat conductive material.Moreover, since layered manufacturing processes can be used to fabricatefeatures, such as internally confined cavities and non-linear orconvoluted passageways, that cannot be fabricated by use of otherprocesses, heat sinks incorporating teachings of the present inventioncan include such features. FIGS. 1-5 illustrate exemplary configurationsof heat sinks incorporating teachings of the present invention.

[0042] With reference to FIG. 1, an assembly is shown that includes asemiconductor device 10 and a heat sink 20 incorporating teachings ofthe present invention. As illustrated, semiconductor device 10 is aflip-chip type semiconductor device, such as a flip-chip die or ballgrid array package, with conductive structures 16 protruding from anactive surface 12 thereof. A thin layer 18 of thermally conductiveadhesive material, such as a suitable epoxy, is disposed on an oppositebackside 14 of semiconductor device 10. The adhesive material of layer18 preferably withstands high temperatures, such as those that willoccur during normal operation of semiconductor device 10.

[0043] Layer 18 secures a heat transfer element 22 of heat sink 20proximate backside 14. Heat transfer element 22 is configured totransfer heat that is generated during use of semiconductor device 10away from semiconductor device 10. Accordingly, it is preferred thatlayer 18 be formed from a material that will readily conduct heat.

[0044] As illustrated, heat transfer element 22 of heat sink 20 haschannels 24 extending therethrough. Channels 24 are configured to permitair to flow through heat transfer element 22 and to thereby facilitatecooling of semiconductor device 10 as the air flowing through channels24 carries heat away, or dissipates heat, from heat transfer element 22.As shown in FIG. 2, channels 24 may be non-linear or convoluted.Channels 24 are preferably configured so as to facilitate the desiredamount of air flow through heat transfer element 22 of heat sink 20 and,thus, to facilitate a desirable level of heat dissipation away fromsemiconductor device 10.

[0045] Heat sink 20 also has a heat dissipation element 26 adjacent heattransfer element 22, opposite semiconductor device 10. Heat dissipationelement 26 includes several upwardly extending fins 28. Fins 28 arespaced apart so as to permit air to flow therebetween and, thus, todissipate heat away from semiconductor device 10.

[0046] FIGS. 3-5 illustrate an assembly that includes a semiconductordevice 10 and another embodiment of a heat sink 20′ incorporatingteachings of the present invention. Heat sink 20′ includes solid heattransfer element 22′ and a heat dissipation element 26′ adjacent heattransfer element 22′, opposite semiconductor device 10.

[0047] Heat dissipation element 26′ includes two sets of fins 30 and 32.Fins 30 are linear and protrude upwardly from heat transfer element 22′.Fins 32 are spaced apart and positioned substantially concentricallyrelative to each other. As shown in FIGS. 3 and 4, spaces 33 betweenadjacent fins 32 are non-linear or convoluted passageways through whichair can flow. Each fin 32 has an upwardly protruding region 34, a bend36, and a laterally extending region 38.

[0048] Turning now to FIG. 10, another embodiment of a heat sink 40according to the present invention is illustrated. Heat sink 40 has aheat transfer element 42 and a heat dissipation element 44. A receptacle46 formed in heat transfer element 42 is configured to receive at leasta portion of a semiconductor device 50. As illustrated, receptacle 42receives a backside 52 and a lower portion of the periphery 54 ofsemiconductor device 50. Receptacle 46 conforms to a portion of thesurface of semiconductor device 50 and contacts the entire backside 52,as well as a portion of the periphery 54 thereof to cup semiconductordevice 50 to facilitate the transfer of heat therefrom to heat sink 40.Heat dissipation element 44, which is remote from semiconductor device50, has spaced apart fins 48 extending therefrom.

Methods of Fabricating Heat Sinks

[0049] In another aspect, the present invention includes methods offabricating heat sinks according to the present invention, such as thoseillustrated in and described with reference to FIGS. 1-5.

[0050] Turning now to FIG. 6, heat sinks 20 according to the presentinvention can be assembled with or fabricated on backsides 14 ofsemiconductor devices 10, such as bare or minimally packagedsemiconductor dice, while semiconductor devices 10 are still part of awafer 72. Each semiconductor device 10 on wafer 72 is separated fromadjacent semiconductor devices 10 by a street 74.

[0051] While the heat sink fabrication process of the present inventionis preferably performed substantially simultaneously on severalsemiconductor devices or other substrates, such as prior to singulatingsemiconductor devices 10 from wafer 72 or on a collection of individualsemiconductor devices or other substrates, such as partial wafers,individual semiconductor devices or other substrates can also beprovided with heat sinks in accordance with teachings of the presentinvention. As another alternative, the method of the present inventioncan be used to substantially simultaneously fabricate heat sinks 20 on acollection of different types of semiconductor devices or othersubstrates.

[0052] The heat sinks of the present invention are preferably fabricatedfrom a thermally conductive material, such as copper, aluminum,tungsten, titanium, or a ceramic material. By way of example and not tolimit the scope of the present invention, the heat sinks can bemanufactured from thermally conductive materials in powdered orparticulate form or in the form of thin sheets.

[0053] For simplicity, the ensuing description is limited to anexplanation of a method of stereolithographically fabricating heat sinks20 directly on semiconductor devices 10 having bare backsides 14. Asshould be appreciated by those of skill in the art, however, the methoddescribed herein is also useful for fabricating heat sinks separatelyfrom a semiconductor device or other substrate, as well as for disposingheat sinks on packaged semiconductor devices or semiconductor deviceshaving one or more layers of protective material on the backsidesthereof. However, the effectiveness of heat transfer from a packaged orcoated device will naturally be somewhat compromised.

Stereolithography Apparatus and Methods

[0054]FIG. 7 schematically depicts various components, and operation, ofan exemplary stereolithography apparatus 80 to facilitate the reader'sunderstanding of the technology employed in implementation of the methodof the present invention, although those of ordinary skill in the artwill understand and appreciate that apparatus of other designs andmanufacture may be employed in practicing the method of the presentinvention. The preferred, basic stereolithography apparatus forimplementation of the method of the present invention, as well asoperation of such apparatus, are described in great detail in UnitedStates patents assigned to DTM Corporation or to Board of Reagents, TheUniversity of Texas System, both of Austin, Tex., or to The B. F.Goodrich Company of Akron, Ohio, such patents including, withoutlimitation, U.S. Pat. Nos. 4,863,538; 4,944,817; 5,017,753; 5,132,143;5,155,321; 5,155,324; 5,156,697; 5,182,170; 5,252,264; 5,284,695;5,304,329; 5,316,580; 5,332,051; 5,342,919; 5,352,405; 5,385,780;5,430,666; 5,527,877; 5,648,450; 5,673,258; 5,733,497; 5,749,041; and5,817,206. The disclosure of each of the foregoing patents is herebyincorporated herein by this reference.

[0055] With continued reference to FIG. 7 and as noted above, a 3-D CADdrawing, in the form of a data file, of an object (e.g., heat sink 20 ofFIGS. 1 and 2) to be fabricated is placed in the memory of a computer 82controlling the operation of apparatus 80, if computer 82 is not a CADcomputer in which the original object design is effected. In otherwords, an object design may be effected in a first computer in anengineering or research facility and the data files transferred via wideor local area network, tape, disc, CD-ROM, or otherwise as known in theart to computer 82 of apparatus 80 for object fabrication.

[0056] The data is preferably formatted in an STL (forSTereoLithography) file, STL being a standardized format employed by amajority of manufacturers of stereolithography equipment. Fortunately,the format has been adopted for use in many solid-modeling CAD programs,so translation from another internal geometric database format is oftenunnecessary. In an STL file, the boundary surfaces of an object aredefined as a mesh of interconnected triangles.

[0057] Data from the STL files resident in computer 82 is manipulated tobuild an object, such as a heat sink 20, illustrated in FIGS. 1 and 2,one layer at a time. Accordingly, the data mathematically representingone or more objects to be fabricated are divided into subsets, eachsubset representing a slice or layer of the object. The division of datais effected by mathematically sectioning the 3-D CAD model into at leastone layer, a single layer or a “stack” of such layers representing theobject. Each slice may be from about 0.003 to about 0.020 inch thick. Asmentioned previously, a thinner slice promotes higher resolution byenabling better reproduction of fine vertical surface features of theobject or objects to be fabricated.

[0058] Apparatus 80 includes a horizontal platform 90 on which an objectis to be fabricated or a substrate disposed for fabrication of an objectthereon. Platform 90 is preferably vertically movable in fine,repeatable increments responsive to computer 82. Material 86 is disposedin a substantially uniform layer of desired thickness by a particulatespreader that operates under control of computer 82. The particulatespreader includes two cartridges 104 a and 104 b disposed adjacentplatform 90 and a roller 102 or scraper bar or blade that is verticallyfixed and horizontally movable across platform 90. As a sufficientquantity of particulate material 86 to form a layer of desired thicknessis pushed upward out of each cartridge 104 a, 104 b by a verticallymovable support 106 a, 106 b, respectively, roller or scraper 102spreads that quantity of particulate material 86 in a uniform layer ofdesired thickness (e.g., 0.003 to 0.020 inches) over platform 90, asubstrate disposed thereon, or an object being fabricated on platform 90or a substrate thereon. Supports 106 a, 106 b of cartridges 104 a, 104 bare preferably vertically movable in fine, repeatable increments undercontrol of computer 82.

[0059] By way of example and not limitation, and as noted above, thelayer thickness of material 86 to be formed, for purposes of theinvention, may be on the order of about 0.003 to 0.020 inch, with a highdegree of uniformity. It should be noted that different material layersmay have different heights, so as to form a structure of a precise,intended total height or to provide different material thicknesses fordifferent portions of the structure.

[0060] With continuing reference to FIG. 7, in a selective lasersintering embodiment of the heat sink fabrication process of the presentinvention, material 86 preferably comprises resin-coated particles ofone or more thermally conductive materials, such as copper, aluminum,tungsten, titanium, ceramics, or a mixture of any of the foregoing,which material 86 is deposited by cartridges 104 a, 104 b and roller orscraper 102 over platform 90 with the latter in its uppermost position.Alternatively, the particles of thermally conductive material may beuncoated, and used alone or mixed with particles of a suitable binderresin.

[0061] A fixative head, such as a laser 92, an ink jet nozzle, or ametal spray gun, is suspended above platform 90. The type of fixativehead employed depends upon the nature of the particulate material 86employed to fabricate the object, as well as an optional binder employedto consolidate particles of material 86 in selected regions of thelayer.

[0062] When the fixative head includes a laser 92, apparatus 80 may alsoinclude a galvanometer 94 with one or more pivotal mirrors. Beforefabrication of a first layer of an object is commenced, the operationalparameters for apparatus 80 are set to adjust the size (diameter, ifcircular) of the laser light beam used to consolidate or fix material86. In addition, computer 82 automatically checks and, if necessary,adjusts by means known in the art the surface level 88 of material 86over platform 90 or a substrate upon which an object is to be fabricatedto maintain same at an appropriate focal length for laser beam 98.Alternatively, the height of the mirror of galvanometer 94 may beadjusted responsive to a detected surface level 88 to cause the focalpoint of laser beam 98 to be located precisely at the surface ofmaterial 86 at surface level 88 if level 88 is permitted to vary,although this approach is more complex.

[0063] The size of the laser beam “spot” impinging on the surface ofmaterial 86 to consolidate or fix same may be on the order of 0.001 inchto 0.008 inch. Resolution is preferably ±0.0003 inch in the X-Y plane(parallel to surface 100) over at least a 0.5 inch×0.25 inch field froma center point, permitting a high resolution scan effectively across a1.0 inch×0.5 inch area. Of course, it is desirable to have substantiallythis high a resolution across the entirety of surface 100 of platform 90to be scanned by laser beam 98, such area being termed the “field ofexposure,” such area being substantially coextensive with the visionfield of a machine vision system employed in the apparatus of theinvention as explained in more detail below. The longer and moreeffectively vertical the path of laser beam 96/98, the greater theachievable resolution.

[0064] The sequence of operation and movements of platform 90,cartridges 104 a, 104 b and their supports 106 a, 106 b, roller 102 orscraper, and laser 92 or another type of fixative head are controlled bycomputer 82.

[0065] Once roller or scraper 102 spreads and smooths material 86 into afirst thin layer 108 of substantially uniform thickness (for example,0.003 to 0.020 inches) over platform 90 or a substrate disposed thereon,laser 92 directs a laser beam 96 toward galvanometer-mounted mirrors 94,which reflect a laser beam 98 toward selected regions of layer 108 inorder to affix the particles of material 86 in the selected regions bymelting or sintering particles of the thermally conductive component ofmaterial 86 or by melting a binder component of material 86 to secureadjacent particles of the thermally conductive component of material 86that are exposed to laser beam 98 to one another. Particles of material86 in these selected regions of layer 108 are preferably affixed in aregular horizontal pattern representative of a first or lowermosttransverse layer or slice of the object to be fabricated, as numericallydefined and stored in computer 82. Accordingly, laser beam 98 isdirected to impinge on particle layer 108 in those areas where thecorresponding layer of the object to be fabricated is comprised of solidmaterial and avoids those areas outside of a periphery of thecorresponding layer of the object to be fabricated, as well as thoseareas of the corresponding layer where a void or aperture exists. Laser98 is withdrawn upon consolidation of material 86 in regions comprisingat least the peripheral outline of the corresponding layer of the objectbeing fabricated.

[0066] With reference to FIG. 9, when material 86 in each of the regionsof layer 108 that correspond to solid areas of the corresponding layerof the object to be fabricated have been exposed to laser beam 98, afirst particle layer 110, or first preform layer, is formed. Firstparticle layer 110 has at least the peripheral outline of thecorresponding layer of the object being fabricated at that vertical orlongitudinal level, material 86 within apertures or voids in layer 110remaining unconsolidated as loose, unfused particles.

[0067] Next, platform 90 is indexed downwardly a vertical distance whichmay or may not be equal to the thickness of the just-fabricated layer110 a (i.e., a layer-manufactured structure may have layers of differentthicknesses). Another layer 110 b of unconsolidated particulate material86 is then formed over layer 110 a as previously described. Laser beam98 is then again directed toward selected regions of the new layer 110 bto follow a horizontal pattern representative of a next, higher layer orslice of the object to be fabricated, as numerically defined and storedin computer 82. As each successive layer 110 is formed by consolidatingmaterial 86 in selected regions, the consolidated material is preferablyalso secured to the immediately underlying, previously fabricated layer110 a. It will be appreciated that, in FIG. 9, the thicknesses of eachlayer 10 has been exaggerated to clearly illustrate the layeredmanufacturing process.

[0068] Of course, since an object to be fabricated by use of astereolithography apparatus, such as apparatus 80, may not haveuniformly configured and sized cross-sections taken transverse to thelength thereof, adjacent layers or slices of the object, whilecontiguous, may not be identical.

[0069] The deposition and smoothing of layers 108 of unconsolidatedparticles of material 86 and the selective fusing of particles ofmaterial 86 in selected regions of each successive layer 108 iscontinued under control of computer 82 for hundreds or even thousands oflayers until a recognizable three-dimensional structure graduallyemerges, and the layering process is further continued until a completedobject has been fabricated. At any time during the fabrication process,or thereafter, unconsolidated particulate material 86 is removed and maybe recovered. Any recovered material may be subsequently used to formanother object.

[0070] As an alternative to the use of a laser to sinter or otherwisebond particles of material 86 in the selected regions of each layer 108together to form layers 110, an ink jet nozzle or a metal spray gun maybe employed as the fixative head. Such a fixative head deposits a liquidbinder (e.g., resin or metal) over the particles of material 86 inselected regions of each layer 108, penetrating therebetween andsolidifying, thus bonding particles in the selected regions of layer 108to at least partially consolidated regions of the next underlying formedlayer 110. If an ink jet nozzle is employed as the fixative head, thebinder may comprise a non-metallic binder such as a polymer compound.Alternatively, when a metal spray gun is used as the fixative head, ametallic binder such as a copper or zinc alloy or Kirksite, aproprietary alloy available through Industrial Modern Pattern and MoldCorp., may be employed. In the case of a metal alloy, the binder may besupplied in wire form which is liquified (as by electric arc heating)and sprayed onto the uppermost particulate layer. Another alternative isto liquify the distal end of the binder wire with a laser or otherheating means immediately above the unconsolidated powder layer ratherthan using a metal spray.

[0071]FIG. 8 illustrates a laminated object manufacturing (LOM)variation of the heat sink fabrication process of the present invention.LOM employs sheets of material to form an object. As depicted in FIG. 8,an apparatus 200 for effecting the LOM method includes a platform 202,actuating means 204 for moving platform 202 in vertical increments, asheet feeder 206, a laser head 208, and a control computer 210. Sheetfeeder 206 may comprise a photocopier-type feeder and provide individualsheets, or may comprise a roll-type feeder with a feed roller and atake-up roller, as desired. In either case, a sheet 212 of suitablematerial, such as a thin metal (e.g., copper, aluminum, tungsten,titanium, etc.) or a ceramic or glass sheet, is placed on platform 202.Laser head 208, under control computer 210, cuts an outline of theperiphery of that layer of the object being fabricated. The surroundingsheet material may then be removed, if desired, and a second, uncutsheet 212′ placed over sheet 212 is bonded to sheet 212 by suitablemeans, after which laser head 208 cuts the perimeter outline of thesecond layer of the object. If desired, the laser may be used to rapidlyheat the second sheet 212′ and bond it to the first sheet 212 beforesheet 212′ is cut at its periphery. Alternatively, a heated roller 214may be biased against and rolled over the uppermost sheet 212′ to securethe uppermost sheet 212′ and the immediately adjacent, underlying sheet212 to each other before the uppermost sheet 212′ is cut to define theperiphery of the corresponding layer of the object being fabricated. Theembodiment of FIG. 8 is particularly suitable for substantiallyconcurrently forming a large plurality of heat sinks on the backside ofan unsingulated semiconductor wafer or other large-scale substrate.

[0072] Such bonding can be effected by melting or sintering, or by anadhesive material disposed on the top, bottom, or both surfaces of eachsheet. One or both surfaces of the sheets may be pre-coated withadhesive, or adhesive may be applied thereto, such as by rolling orspraying, during the layered manufacturing process.

[0073] Referring again to FIG. 7, in practicing the present invention, acommercially available stereolithography apparatus operating generallyin the manner as that described above with respect to apparatus 80 ispreferably employed, but with further additions and modifications ashereinafter described for practicing the method of the presentinvention. For example and not by way of limitation, the SINTERSTATION®2000, SINTERSTATION® 2500, and SINTERSTATION® 2500 plusstereolithography systems, each offered by DTM Corporation of Austin,Tex., are suitable for modification.

[0074] It should be noted that apparatus 80 useful in the method of thepresent invention includes a camera 140 which is in communication withcomputer 82 and preferably located, as shown, in close proximity togalvanometer 94 located above surface 100 of support platform 90. Camera140 may be any one of a number of commercially available cameras, suchas capacitive-coupled discharge (CCD) cameras available from a number ofvendors. Suitable circuitry as required for adapting the output ofcamera 140 for use by computer 82 may be incorporated in a board 142installed in computer 82, which is programmed as known in the art torespond to images generated by camera 140 and processed by board 142.Camera 140 and board 142 may together comprise a so-called “machinevision system” and, specifically, a “pattern recognition system” (PRS),operation of which will be described briefly below for a betterunderstanding of the present invention. Alternatively, a self-containedmachine vision system available from a commercial vendor of suchequipment may be employed. For example, and without limitation, suchsystems are available from Cognex Corporation of Natick, Mass. Forexample, the apparatus of the Cognex BGA Inspection Package or the SMDPlacement Guidance Package™ may be adapted to the present invention,although it is believed that the MVS-8000™ product family and theCheckpoint™ product line, the latter employed in combination with CognexPatMax™ software, may be especially suitable for use in the presentinvention.

[0075] It is noted that a variety of machine vision systems are inexistence, examples of which and their various structures and uses aredescribed, without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659;4,736,437; 4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174;5,463,227; 5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and5,644,245. The disclosure of each of the immediately foregoing patentsis hereby incorporated by this reference.

[0076] Of course, apparatus 200 depicted in FIG. 8 could also beequipped with such a machine vision.

Stereolithographic Fabrication of the Heat Sinks

[0077] Referring now to FIGS. 7 and 9, in order to facilitatefabrication of one or more heat sinks 20 in accordance with the methodof the present invention with apparatus 80, a data file representativeof the size, configuration, thickness and surface topography of, forexample, a particular type and design of semiconductor device 10 orother substrate upon which one or more heat sinks 20 are to befabricated is placed in the memory of computer 82. Also, it may bedesirable to place a data file representative of the various features ofsemiconductor device 10 in memory.

[0078] One or more semiconductor devices 10, wafers 72, or othersubstrates may be placed on surface 100 of platform 90 to have heatsinks 20 fabricated thereon. Camera 140 is then activated to locate theposition and orientation of each semiconductor device 10, includingthose on a wafer 72, or other substrate. The features of eachsemiconductor device 10, wafer 72, or other substrate are compared withthose in the data file residing in memory, the locational andorientational data for each semiconductor device 10, wafer 72, or othersubstrate then also being stored in memory. It should be noted that thedata file representing the design size, shape and topography for eachsemiconductor device 10 or other substrate may be used at this junctureto detect physically defective or damaged semiconductor devices 10 orother substrates prior to fabricating a heat sink 20 thereon or beforeconducting further processing or assembly of semiconductor device 10 orother substrates. Accordingly, such damaged or defective semiconductordevices 10 or other substrates can be deleted from thestereolithographic heat sink fabrication process, from furtherprocessing, from further testing, or from assembly with othercomponents. It should also be noted that data files for more than onetype (size, thickness, configuration, surface topography) ofsemiconductor device 10 or other substrate may be placed in computermemory and computer 82 programmed to recognize not only the locationsand orientations of each semiconductor device 10 or other substrate, butalso the type of semiconductor device 10 or other substrate at eachlocation upon platform 90 so that material 86 may be at least partiallyconsolidated by laser beam 98 in the correct pattern and to the heightrequired to fabricate heat sinks 20 in the appropriate, desiredlocations on each semiconductor device 10 or other substrate.

[0079] Continuing with reference to FIGS. 7 and 9, a substantiallyuniform layer 108 of material 86 is disposed over wafer 72 or the one ormore semiconductor devices 10 or other substrates on platform 90 to adepth substantially equal to the desired thickness of a formed layer 110of heat sink 20.

[0080] Laser 92 is then activated and scanned to direct beam 98, undercontrol of computer 82, toward specific locations of surface 88 relativeto each semiconductor device 10 or other substrate to effect theaforementioned partial cure of material 86 to form a first layer 110 aof each heat sink 20. Platform 90 is then lowered and another layer 108of material 86 of a desired thickness disposed over formed layer 110 a.Laser 92 is again activated to add another layer 110 b to each heat sink20 under construction. This sequence continues, layer by layer, untileach of the layers 110 of each heat sink 20 have been completed.

[0081] In FIG. 9, the first, bottommost layer of heat sink 20 isidentified by numeral 110 a, and the second layer is identified bynumeral 110 b. As illustrated, heat sink 20 has only a few layers 110.In practice of the invention, however, heat sinks 20 will often havemany thin layers 110. Accordingly, heat sinks 20 with any number oflayers 110 are within the scope of the present invention.

[0082] Each layer 110 of heat sink 20 may be built by first defining anyinternal and external object boundaries of that layer with laser beam98, then hatching solid areas of that layer of heat sink 20 locatedwithin the object boundaries with laser beam 98. An internal boundary ofa layer may comprise a portion of a channel 24, a space between adjacentfins 32 (see FIGS. 3-5), a through-hole, a void, or a recess in heatsink 20, for example. If a particular layer includes a boundary of avoid in the object above or below that layer, then laser beam 98 isscanned in a series of closely-spaced, parallel vectors so as to developa continuous surface, or skin, with improved strength and resolution.The time it takes to form each layer 110 depends upon the geometrythereof, the surface tension and viscosity of material 86, and thethickness of that layer.

[0083] Once heat sinks 20 have been fabricated, platform 90 is elevatedand removed from apparatus 80, along with any substrate (e.g.,semiconductor device 10, wafer 72 (see FIG. 6), or other substrate)disposed thereon and any stereolithographically fabricated structures,such as heat sink 20. Excess, unconsolidated material 86 (e.g., excesspowder or particles) may be manually removed from platform 90, from anysubstrate disposed thereon, and from heat sink 20. Each semiconductordevice 10, wafer 72, or other substrate is removed from platform 90.

[0084] Residual particles of the thermally conductive material that wasused to fabricate heat sink 20 are preferably removed by use of knownsolvents or other cleaners that will not substantially degrade, deform,or damage heat sink 20 or the substrate (e.g., semiconductor device 10)on which heat sink 20 was fabricated. Such cleaning is particularlyimportant when electrically conductive materials, such as copper,aluminum, tungsten, or titanium, are used to fabricate heat sink 20, asa residue of such electrically conductive materials can cause electricalshorts that will result in failure of semiconductor device 10.

[0085] Although FIGS. 7-9 illustrate the stereolithographic fabricationof heat sink 20 on a substrate, such as a semiconductor device 10, awafer 72, or another substrate, heat sink 20 can be fabricatedseparately from a substrate, then secured thereto by known processes,such as by the use of a suitable adhesive material.

[0086] The use of a stereolithographic process as exemplified above tofabricate heat sink 20 is particularly advantageous since a large numberof heat sinks 20 may be fabricated in a short time, the dimensions andpositions thereof are computer controlled to be extremely precise,wastage of construction material 86 is minimal, and thestereolithography method requires minimal handling of semiconductordevices 10, wafers 72, or other substrates.

[0087] Stereolithography is also an advantageous method of fabricatingheat sinks according to the present invention since, when resinousbinders are used to secure adjacent particles of thermally conductivematerial in selected regions, stereolithography can be conducted atsubstantially ambient temperature, the small spot size and rapidtraverse of laser beam 98 resulting in negligible thermal stress uponsemiconductor devices 10, wafers 72, or other substrates, as well as onthe features thereof.

[0088] The stereolithography fabrication process may also advantageouslybe conducted at the wafer level or on multiple substrates, savingfabrication time and expense. As the stereolithography method of thepresent invention recognizes specific semiconductor devices 10 or othersubstrates, variations between individual substrates are accommodated.Accordingly, when the stereolithography method of the present inventionis employed, heat sinks 20 can be simultaneously fabricated on differenttypes of semiconductor devices 10 or other substrates, as well as onboth semiconductor devices 10 and other substrates.

[0089] Stereolithography may also be used to form a wafer-level array ofheat sinks separately from a semiconductor wafer, each heat sink of thearray corresponding to a semiconductor device of the wafer. These heatsinks can be bonded to a wafer, then the wafer separately singulatedwith the heat sinks being simultaneously singulated.

[0090] While the present invention has been disclosed in terms ofcertain preferred embodiments, those of ordinary skill in the art willrecognize and appreciate that the invention is not so limited.Additions, deletions and modifications to the disclosed embodiments maybe effected without departing from the scope of the invention as claimedherein. Similarly, features from one embodiment may be combined withthose of another while remaining within the scope of the invention.

What is claimed is:
 1. A heat sink for assembly with a semiconductor device component, comprising: a heat transfer element configured to be secured to the semiconductor device and including at least one non-linear passageway therethrough.
 2. The heat sink of claim 1, wherein at least a portion of said heat transfer element comprises a plurality of superimposed, contiguous, mutually adhered layers of thermally conductive material.
 3. The heat sink of claim 2, wherein said thermally conductive material comprises a metal.
 4. The heat sink of claim 3, wherein said metal comprises copper, aluminum, tungsten, or titanium.
 5. The heat sink of claim 2, wherein said thermally conductive material comprises a ceramic or a glass.
 6. The heat sink of claim 1, wherein said heat transfer element comprises a plurality of particles that are secured to one another.
 7. The heat sink of claim 6, wherein adjacent ones of said particles are sintered together.
 8. The heat sink of claim 6, wherein adjacent ones of said particles are secured together with a binder.
 9. The heat sink of claim 2, wherein at least some of said plurality of superimposed, contiguous, mutually adhered layers comprise sheets of said thermally conductive material.
 10. The heat sink of claim 9, wherein adjacent sheets are secured together with an adhesive material.
 11. The heat sink of claim 9, wherein adjacent sheets are thermally bonded together.
 12. The heat sink of claim 1, wherein said at least one non-linear passageway is configured to permit airflow therethrough.
 13. The heat sink of claim 1, further comprising a heat dissipation element adjacent to said heat transfer element and extending to a location remote from the semiconductor device.
 14. The heat sink of claim 13, wherein at least a portion of said heat dissipation element comprises a plurality of superimposed, contiguous, mutually adhered layers of thermally conductive material.
 15. The heat sink of claim 14, wherein said heat dissipation element includes a plurality of fins. 